Effect of Powder Filling Rate on Microstructure and Wear Resistance of Composite Powder and Solid Wire Cladding Alloys

Literature Overview and Process Context

This study investigates the influence of powder filling rate on the microstructure, phase composition, and tribological performance of cladding alloys produced by a hybrid technique combining composite powder and solid wire feeding. The hybrid feeding approach is designed to overcome the individual limitations of powder-only and wire-only cladding processes, leveraging the compositional flexibility of composite powders and the deposition efficiency of solid wires. The powder filling rate, defined as the volumetric or mass rate at which powder is delivered to the weld pool relative to the wire feed rate, emerges as a critical process parameter governing the final deposit properties.

The practical motivation for this research is clear: industrial cladding applications often require deposits with simultaneously high hardness, good toughness, and controlled dilution, which is difficult to achieve with a single feedstock. By adjusting the powder-to-wire ratio and the powder filling rate, engineers can tune the deposit composition and microstructure to meet specific performance requirements.

Microstructural Evolution with Powder Filling Rate

The study reveals a systematic evolution of microstructure as the powder filling rate increases from 0% (pure wire) to 100% (pure powder). At low powder filling rates (0-20%), the microstructure is dominated by the solidification pattern of the wire material, with columnar grains growing from the substrate interface into the deposit. As the powder filling rate increases to 40-60%, the powder particles act as heterogeneous nucleation sites, promoting equiaxed grain formation and grain refinement. At high powder filling rates (80-100%), the powder particles become so numerous that they significantly alter the local thermal gradient and solidification rate, producing a fine-grained microstructure with complex phase distributions.

The phase evolution follows a predictable pattern based on the Schaeffler diagram and phase equilibrium considerations. At low powder rates, the wire composition dominates, and the deposit solidifies as austenite or austenite-ferrite duplex. As the powder content increases, the carbon and alloy element concentrations rise, promoting ferrite and martensite formation. The appearance of M2B (iron-boron carbide) phases at high powder rates indicates that the composite powder contains boron-containing particles, which form hard, wear-resistant but potentially brittle phases.

Wear Resistance and Tribological Performance

Wear testing was conducted using a pin-on-disk tribometer under dry sliding conditions. The results demonstrate a clear correlation between powder filling rate, microstructure, and wear resistance. The wear rate decreases monotonically with increasing powder filling rate, from approximately 1.5-2.0 × 10⁻⁶ mm³/N·m for pure wire deposits to 0.3-0.5 × 10⁻⁶ mm³/N·m for pure powder deposits.

The primary wear mechanism transitions from adhesive wear at low powder rates (where the softer matrix material deforms and transfers to the counterface) to abrasive wear at high powder rates (where hard carbide particles plough the counterface). The transition point occurs at approximately 40% powder filling rate, where the microstructure becomes sufficiently hard and refined to resist adhesive transfer.

An interesting observation is that the wear rate does not decrease indefinitely with increasing powder filling rate. Beyond 80%, the marginal improvement in wear resistance diminishes because the increased brittleness of the martensitic matrix and the presence of coarse M2B phases can lead to microcracking and particle pullout, which partially offsets the benefits of higher hardness.

Process Optimisation and Dilution Control

A critical practical consideration is the dilution rate, which represents the fraction of base material incorporated into the deposit. The powder filling rate directly influences the dilution rate because powder particles add mass and heat absorption to the weld pool, reducing the relative contribution of the base material.

The dilution rate decreases from approximately 40% at zero powder filling to about 8% at 100% powder filling. This reduction in dilution is beneficial for maintaining the intended deposit composition, but it also means that the base material contribution to the deposit properties is reduced, which can affect the metallurgical bonding at the interface.

The bond strength shows a non-monotonic relationship with powder filling rate, peaking at approximately 40% powder filling rate. This optimum corresponds to the balance between sufficient heat input for good interface fusion (which requires base material melting) and adequate dilution to create a graded transition zone. At very high powder rates, the reduced dilution can lead to incomplete fusion at the interface, weakening the bond.

Engineering Practice Implications and Defect Analysis

For industrial applications, the selection of powder filling rate must balance multiple competing requirements. High powder rates produce harder, more wear-resistant deposits but with reduced toughness and potential brittleness. Low powder rates produce tougher, more ductile deposits but with lower hardness and wear resistance. The optimal powder filling rate depends on the specific service conditions.

Common defects associated with powder feeding include powder bounce (particles rebounding from the substrate without being incorporated), powder segregation (uneven distribution of powder particles within the deposit), and excessive porosity (trapped gas from powder decomposition or incomplete melting of powder particles). The powder filling rate directly influences these defects: too high a rate can overwhelm the arc's ability to melt and incorporate all particles, leading to bounce and porosity.

The recommended maximum powder filling rate for stable processing is typically 60-80% of the total feed mass, with the remainder supplied as solid wire. This configuration provides the best balance between deposition efficiency, deposit quality, and process stability.

Study Insights and Practical Recommendations

The most significant finding of this study is the existence of an optimal powder filling rate window (approximately 40-60%) that maximises the combination of wear resistance, toughness, and bond strength. This finding has direct implications for process development in hybrid powder-wire cladding systems, where the feedstock ratio can be adjusted to tailor the deposit properties to specific service requirements.

The study also highlights the importance of considering the full property set (hardness, toughness, wear resistance, bond strength, dilution) rather than optimising for a single property. The temptation to maximise hardness by using high powder rates must be resisted when the application requires good toughness or when the base material has limited ability to accommodate the thermal and mechanical demands of the overlay process.

For engineers implementing hybrid powder-wire cladding in production, the following practical recommendations emerge: (1) start with a powder filling rate of 40-50% and adjust based on the required deposit properties; (2) monitor the dilution rate through chemical analysis of the deposit cross-section; (3) conduct wear testing under conditions representative of the actual service environment; and (4) maintain process parameters within the established stability window to avoid defects.

In summary, the powder filling rate is a powerful process variable that enables systematic control of cladding deposit microstructure and performance, and the established relationships between powder rate, microstructure, and wear resistance provide a rational basis for process optimisation in hybrid powder-wire cladding applications.